In css w3(here),
1px = 0.75pt, 1pt = 2.54/72 cm, so 1 css pixel:1px = 0.26mm
for example, the CSS pixel of iPhone4 is 320px * 480px(not device pixel),
so in width ,the length should be:
320 *0.26 = 83.2mm
but the iPhone4 is 3.5"，so the length in width is about 60mm,
it dont match the result i calculated above
where is wrong?
thanks
The physical unit is based on 96dpi, therefore 1in in css is 96px, so 3.5in = 336px. Here's a blog post which can explain this: http://www.quirksmode.org/blog/archives/2012/11/the_css_physica.html but it's also mentioned in the W3 spec you posted if you scroll down a bit.
The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length.
A CSS pixel is not always 0.26mm
From CSS W3:
The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length. For a nominal arm's length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm's length, 1px thus corresponds to about 0.26 mm (1/96 inch).
The iPhone4 is not 96dpi. With a device pixel ratio of 2, its "CSS dpi" is around 163. Since 96dpi devices render 320px at 83.2mm, mathematically, 163dpi devices should render 320px at 49mm.
Related
What is a point? whereas pixel is clear enough: it is a physical unit on the screen, the nature of the point in not so explicit.
A point is a measure that equals 1/72 inches. The main difference between pointSize and pixelSize is that pointSize is density independent, which means that the size is physically fixed, whatever screen you use.
Highdpi scaling can or can not affect point size, it depends on the setup: https://doc.qt.io/qt-5/highdpi.html.
If I am correct, the point size is based on an abstract layer provided by Qt to do a High DPI scaling.
Please follow:
https://doc.qt.io/qt-6/highdpi.html
there are 160 units per inch.
2.If I created an Photoshop file that are 72 dpi then there will be 72 points per inch.
3.If the element is 88px height in Photoshop then what I have to set it in xamarin?
If the phone is 360dpi then the height in xamarin should be :88 / 72 * 160 / 2?
but it is not right.
I know the units in each platfform and I only want to know how to get units from pixels
You could use Xamarin.Essentials to get the Screen density as follows:
// Get Metrics
var mainDisplayInfo = DeviceDisplay.MainDisplayInfo;
// Screen density
var density = mainDisplayInfo.Density;
If the density is 3 in iOS device and pixels is 88 , then there are 88/3 units in iOS.
If the phone is 360dpi
That means it shoule be a Android device, and screen density also can be calculated by 360/160 = 2.25. Then there are 88/2.25 units to set for HeightRequest.
================================Update==============================
If there is a BoxView in Xaml as follows:
<BoxView x:Name="MyBoxView" BackgroundColor="CadetBlue" HorizontalOptions="Fill"/>
And the effect:
Now I will print the MyBoxView.Width. the result is:
Console.WriteLine("++++MyBoxView++++" + MyBoxView.Width);
++++MyBoxView++++411.428571428571
If you use var density = mainDisplayInfo.Density; to get density, you will get Screen Density::2.625. (My device is Piexl_2_pie_9_0 api 28 emulator)
You know the size of screen width is 1080 pixels, however the width is 411.428571428571. That should means units of WidthRequest.
And if you put 411.428571428571 * 2.625 , you will get 1080 pixels.
It sounds simple but how is the fontsize, which can be set with setPointSize defined in Qt4? A point is 1/72 inch but if I compare to
glyph metrics from FreeType
Is it the max_advance_height, the advance between two lines or the distance between the highest and lowest point (the maximum ascender - descender)?
If I create a QLabel and setPointSize to 75 I get the following result:
My screen has 96x96 dpi which should result in 75/72*96 = 100 pixels. But measured the B, for example is 70 pixels in height.
I'm developing a psychology experiment in the browser. In order to keep the same viewing angle across people, I want to display two characters around 5 inches apart on the screen.
Is there any way to detect the real size of the monitor being used, and using the screen resolution and DPI, render the two objects the same real width apart? (I will only allow people that have real computers, e.g. not mobile)
I heard detecting real size may not be possible, if true, and assuming people will report to me the real size of their monitor, is this possible?
I'm using HTML5 Canvas, fwiw. Perhaps resizing this canvas w.r.t to the resolution and DPI is a solution.
No, unfortunately. The browser will always report 96 DPI. Without actual DPI you cannot produce exact measures in other units than pixels.
Even if you could the browser would only reflect the system DPI which in itself is just an approximation.
You need to "calibrate" for the individual device providing a mechanism to do so, e.g. a scale that can be varied and measure on screen. When it measures 1 inch you know how many pixels covers that inch, and then this value can be used as a scale for the rest.
Example on how to get screen DPI via "calibration"
var ctx = document.querySelector("canvas").getContext("2d"),
rng = document.querySelector("input");
ctx.translate(0.5, 0.5);
ctx.font = "16px sans-serif";
ctx.fillStyle = "#c00";
render(+rng.value);
rng.onchange = rng.oninput = function() {render(+this.value)}; // update on change
function render(v) {
ctx.clearRect(-0.5, -0.5, 600, 300);
ctx.strokeRect(0, 0, v, v);
ctx.fillText(v + " PPI", 10, 20);
// draw marks which should be 4 inches apart
ctx.fillRect(0, 0, 3, 150);
ctx.fillRect(96*4 * (v / 96), 0, 3, 150); // assuming 96 DPI base resolution
ctx.fillText("------ Should be 4 inches apart ------", 50, 140);
}
<label>Adjust so square below equals 1 inch:
<input type=range value=96 min=72 max=145></label>
<canvas width=600 height=300></canvas>
This example can of course be extended to take a vertical parameter as well as considering pixel aspect ratio (ie. retina displays) and scale.
You need to then build all your objects and graphics using a base scale, for example 96 DPI. Then use the relationship between the actual DPI and 96 DPI as a scale factor for all positions and sizes.
I am not asking what is the difference between dp,sp and px.
I am designing a website based on google's new material design, all the measurements are in dp (for grid) and sp(for text). My question is how do they translate to pixels. I have been designing websites for more than 4 years and all the measurements (grid and font) are in pixels.
For Example:
A headline is 24sp, how many pixels does it equate to? (its not 24px, I've tried matching them, it's around 28px but there has to be a standard measuring systems).
Grid Guideline: "All components align to an 8 dp square baseline grid." - how many pixels does it equate to?
1px = ?dp =?sp on a desktop or any average monitor or mobile device?
I recommend reading Google's definitions of dp and sp, which can be found in the Android docs, here and here.
There's also some helpful information in the wonderful Designer's Guide to DPI.
I think the answer is going to be:
1px = 1dp = 1sp on any average monitor or mobile device.
How did I come up with this?
Because a pixel is a pixel, for andriod dp and sp are used because they are used for native apps which have to scale and the dpi of each screen is different based on device. For desktops all of this is same, off course the website has to be compatible/responsive for mobile devices but since the website loads in a browser, some additional media quires (based on guidelines) will do the job.
If anybody has some other logical conclusion, please share
A safe rule of thumb is to use 1 px = 1 dp.
This should give you a good safe size on just about any device. It will appear a bit large on some devices, notably the iPad (regular).
Here's why:
"A dp corresponds to the physical size of a pixel at 160 dpi" (https://developer.android.com/training/multiscreen/screendensities.html#TaskUseD)
160 dpi means:
160 dots = 1 inch
Therefore:
160 dp = 1 inch (25.4 mm)
So when Google recommends that buttons have a touchable target height of 48 dp, they're saying that they need to be 0.3 inches (7.6 mm) tall.
So how many px is this? Well, that depends on the device.
Examples for 48 dp (7.6 mm) button height:
iPad mini: 48 px Why: The iPad mini screen is about 120 mm wide and uses 768 px to fill that space. You therefore need 162 px to take up an inch (25.4 mm), or 48 px for your button height of 7.6 mm.
Kindle Fire (7"): 43 px
Kindle Fire (6"): 50 px
iPhone: 48 px
Nexus 7: 48 px
Regular iPad: 39 px
(I may have fudged the rounding up/down a tiny bit.. I like 48 better than 49!)
Screen mm and CSS px width for examples: I calculated the screen width using the CSS px screen dimensions and diagonal length.
iPad mini: 1024 x 768 resolution and 201 mm diagonal = 120 mm width.
Kindle Fire 7": 858 x 533 resolution and 178 mm diagonal = 94 mm width.
Kindle Fire 6": 853 x 533 resolution and 152 mm diagonal = 81 mm width.
iPhone: 568 x 320 resolution and 102 mm diagonal = 50 mm width.
Nexus 7: 960 * 600 resolution and 178 mm diagonal = 94 mm width.
iPad regular: 1024 x 768 resolution and 246 mm diagonal = 148 mm width.
Note that for calculating the px height of the button you need to use the device CSS px dimensions. These numbers are not necessarily the same as the resolutions stated in the specs.
All are roughly equivalent for most use cases.
Source
To preserve the visible size of your UI on screens with different densities, you must design your UI using density-independent pixels (dp) as your unit of measurement. One dp is a virtual pixel unit that's roughly equal to one pixel on a medium-density screen (160dpi; the "baseline" density). Android translates this value to the appropriate number of real pixels for each other density.
For example, consider the two devices in figure 1. If you were to define a view to be "100px" wide, it will appear much larger on the device on the left. So you must instead use "100dp" to ensure it appears the same size on both screens.
When defining text sizes, however, you should instead use scalable pixels (sp) as your units (but never use sp for layout sizes). The sp unit is the same size as dp, by default, but it resizes based on the user's preferred text size.
Figure 1. Two screens of the same size may have a different number of pixels